Resistance to established drugs for cancer therapy is in many cases associated with overexpression of the multidrug resistance gene 1 (MDR-1). Regulation of basal expression of MDR-1 and mechanisms of induction as a result of exposure to cytotoxic substances are still not completely understood. Recent reports have suggested an association of the C3435T polymorphism in exon 26 of the MDR-1 gene with MDR-1 expression in duodenal mucosa cells of healthy individuals. We analyzed the C3435T and G2677T genotypes of 38 mammary and ovarian carcinoma cell lines and measured basal MDR-1 expression by real-time reverse transcriptase-polymerase chain reaction. Cell lines were classified as non-expressing or showing weak basal expression that was found to be significantly associated (six/seven versus 13/31 expressing cell lines; P=0.0448, Fisher's exact test) with the TT genotype at position 3435 of the MDR-1 gene.